1. Field of the Invention
The present invention relates to a converter transformer used in a switching regulator or the like employing the circuit of a flyback converter.
2. Description of the Prior Art
As shown in FIG. 6, the circuit of a flyback converter of a general type has a converter transformer T, an input rectifying circuit BR1 for rectifying an input voltage Vin, a fuse F1, a drive-control and overvoltage protection circuit CNT1, a photo coupler consisting of a light emitting element PC1-2 and a light receiving element PC1-1, a main switching element Q1, an output detecting circuit REF1, resistors R1 to R7, capacitors C1 to C6, diodes D1 to D4, an inductor L1, and the like.
This transformer T has primary windings consisting of a primary excitation winding P1 and a primary auxiliary winding P2 and secondary windings consisting of a secondary winding S1 for producing a main operating voltage and another secondary winding S2 for producing another operating voltage.
In operation, the AC input Vin is rectified by the input rectifying circuit BR1 and is smoothed by the capacitor C1 to be converted to a DC voltage. This DC voltage is the input voltage of the converter transformer.
The capacitor C4 is charged by a starting current flowing through the starting resistor R1 to actuate the drive-control and overvoltage protection circuit CNT1. Then, the main switch Q1 is turned on, and an input voltage is impressed on the primary excitation winding P1 of the converter transformer T. In this state, the diodes D2, D3 and D4 are turned off, and all energy supplied to the primary excitation winding P1 is accumulated in the converter transformer T.
Then, the main switching element Q1 is turned off, and the diodes D2, D3 and D4 are turned on. The energy accumulated in the converter transformer T is supplied from the primary auxiliary winding P2 to the power source voltage side of the drive-control and overvoltage protection circuit CNT1 and is discharged from the secondary winding S1 which produces the main operating voltage and another secondary winding S2 which produces another operating voltage to the secondary output side.
The on/off drive control of the main switching element Q1 is made in the following way. The output detecting circuit REF1 connected to the output side of the secondary winding S1 which produces the main operating voltage detects the output voltage of the secondary winding S1 which produces the main operating voltage. The detected signal is fed back to the drive-control and overvoltage protection circuit CNT1 through the photo coupler consisting of the light emitting element PC1-2 and the light receiving element PC1-1. The fed back signal from the drive-control and overvoltage protection circuit CNT1 controls the drive of the main switching element Q1 connected to the primary excitation winding P1 so that the output voltage of the secondary winding Si which produces the main operating voltage is stabilized.
The conventional converter transformer which operates by means of a flyback converter circuit or the like has a structure as shown in FIGS. 7A and 7B or FIGS. 8A and 8B. In FIGS. 7A and 7B are shown a cross-sectional view and a circuit diagram of one of the conventional converter transformers, respectively. The transformer includes a bobbin 2 having a magnetic core 1 inserted therein and a plurality of flanges 2a to 2d formed thereon. Chambers 3a to 3c are individually defined between the adjacent flanges 2a to 2d on the bobbin 2. The primary excitation windings consist of a primary excitation winding P1-1 wound on a chamber 3a and another primary excitation winding P1-2 wound on another chamber 3c. The primary auxiliary winding P2 which is one of the primary windings is wound on the outer periphery of the primary excitation winding P1-1. The second windings S1 and S2 are wound in an overlapping manner on the chamber 3b between the chambers 3a and 3c on which the primary excitation windings P1-1 and P1-2 are wound individually. This arrangement ensures the electrical insulation satisfying the safety standard.
When the leakage inductance of the primary windings P1 (P1-1 and P1-2) and P2 and the secondary windings S1 and S2 is large, large spike voltage is produced in the windings P1, P2, S1 and S2 upon turning the main switching element Q1 off. In order to make the leakage inductance as small as possible, the primary excitation windings P1-1 and P1-2 are individually wound on the different chambers 3a and 3c, and the secondary windings S1 and S2 are wound on the chamber 3b between the chambers 3a and 3c, as described above.
In FIGS. 8A and 8B are shown a cross sectional view and a circuit diagram of another conventional converter transformer. In this transformer, the primary windings P1 and secondary windings S1 and S2 are divided into the primary windings P1-1, P1-2 and P1-3 and the secondary windings S1-1, S1-2, S2-1, and S2-2 which are more than those of the transformer as shown in FIGS. 7A and 7B. The primary windings P1-1, P1-2 and P1-3 are connected in series and wound on every other chambers 3a, 3c and 3e, respectively. A pair of the secondary windings S1-1 and S1-2 are connected in series, whereas another pair of the secondary windings S2-1 and S2-2 are connected in parallel. The secondary windings S1-1 and S1-2 are wound on the chamber 3b between the chambers 3a and 3c and on the chamber 3d between the chambers 3c and 3e, respectively. Further, the secondary windings S2-1 and S2-2 are wound around the secondary winding S1-1 on the chamber 3b and around the secondary winding S1-2 on the chamber 3d, respectively. The primary auxiliary winding P2 is wound on the outer periphery of one of the so-divided primary windings P1-2.
In these conventional converter transformers, the primary auxiliary winding P2 is an important winding which provides an electric power to the drive-control and overvoltage protection circuit CNT1 to control the main switching element Q1 as shown in FIG. 6. When, therefore, the spike voltage Vs produced in the primary auxiliary winding P2 upon turning the main switching element Q1 off is large as shown in FIG. 9, the fly-back converter circuit cannot be controlled stably with the result that the main switching element Q1 cannot operate suitably or the overvoltage protection circuit portion assembled in the fly-back converter circuit operates erroneously.
In order to suppress the spike voltage produced in the primary auxiliary winding P2 in the conventional converter transformer, it is necessary that the inductor L1, the resistor R4 and the like be connected in series to the primary auxiliary winding P2, or a snubber circuit consisting of the resistor R3 and the capacitor C3 be connected in parallel to the primary auxiliary winding P2.